With the recent rapid spread of the Internet, access service systems require high capacity, high functionality, and cost efficiency, and research on WDM/TDM-PON is being conducted as means for achieving such systems. A passive optical network (PON) is an optical communication system in which a plurality of subscriber apparatuses (optical network units (ONUs)) share a single station-side subscriber accommodation apparatus (an optical line terminal (OLT)) and part of a transmission path using an optical multiplexer/demultiplexer composed of optical passive elements, thereby reducing cost.
At present, in Japan, a gigabit Ethernet passive optical network (GE-PON) (Ethernet is a registered trademark), which is an economic optical subscriber system in which a maximum of 32 users share a line capacity of 1 gigabits per second (Gbps) in accordance with time division multiplexing (TDM), is mainly introduced. Thereby, a fiber to the home (FTTH) service is provided at a reasonable fee.
Also, research on 10G-EPON in which the total bandwidth is in a 10 Gbps class is being conducted as the next-generation optical subscriber system in order to cope with the need for larger capacities, and the international standardization thereof was completed in 2009. This is an optical subscriber system for achieving a large capacity while using the same transmission path, such as optical fibers, as that of the GE-PON by increasing the bit rate of the transceiver.
Further, it is considered that a large capacity exceeding the 10G class for ultra-high-definition video services and ubiquitous services is required in the future, but there is a problem in that even if the bit rate of the transceiver is simply increased from the 10G class to the 40/100G class, practical application is difficult because of an increase of cost required to upgrade the system.
As means for solving this problem, a wavelength-tunable WDM/TDM-PON in which wavelength tunability is added to a transceiver and time division multiplexing (TDM) is effectively combined with wavelength division multiplexing (WDM) so that the transceiver in the OLT can be upgraded step by step in accordance with a required bandwidth amount has been reported (e.g., see Non-Patent Document 1).
As in Non-Patent Document 2, the wavelength-tunable WDM/TDM-PON system has recently attracted attention as a system in which a stepwise upgrade of the total bandwidth and flexible load balancing in accordance with demands of users are possible, and a dynamic wavelength bandwidth allocation algorithm is used to change line cards (optical subscriber units (OSUs)) belonging to the OLT in accordance with load balancing at the time of the stepwise upgrade of the total bandwidth. Dynamic wavelength and bandwidth allocation (DWBA) is achieved by combining upstream dynamic bandwidth allocation (DBA) from the ONUs in the belonging OSUs with wavelength switching for switching the belonging OSUs.
FIGS. 1, 2, and 12 show the configuration of a wavelength-tunable WDM/TDM-PON system and the configurations of a station-side subscriber accommodation apparatus (OLT) 100 and subscriber apparatuses (ONUs) 200, which configure the system. The OLT 100 and the ONUs 200 are connected with PON topology of a point-to-multipoint configuration using power splitters or wavelength routers 140 and 150.
In FIG. 1, the OLT 100 includes OSUs 120 of line cards OSU #1 to OSU #m for transmitting and receiving signals of wavelength sets of λ1d,u to λmd,u, a dynamic wavelength bandwidth allocation circuit 110, and a multiplexing/demultiplexing unit 130. The OSUs 120 of OSU #1 to OSU #m transmit and receive the signals of the wavelength sets of λ1d,u to λmd,u to and from the ONUs 200. The h ONUs 200 of ONU #1 to ONU #h are connected to the OLT 100 and each ONU 200 performs transmission and reception using one of the wavelength sets of λ1d,u to λmd,u, each of which is a set of a downstream wavelength and an upstream wavelength. The ONUs 200 can perform transmission and reception while switching signals of wavelength sets of λ1d,u to λmd,u in accordance with an instruction from the OLT 100.
An upstream signal from a communication apparatus (not shown) in a user's home in which each ONU 200 is installed is input to each ONU 200 and the upstream signal is transmitted as an upstream optical signal by an optical transceiver within each ONU 200. Upstream signals are multiplexed into one optical fiber from the power splitter or the wavelength router 150 on the ONU 200 side toward the OLT 100. Thus, the OLT 100 calculates and controls the transmission time and the transmission duration of the upstream signal transmitted by each ONU 200 so that the upstream signals do not overlap with each other. Upstream signals 1 to m received by the OSUs 120 of OSU #1 to OSU #m are aggregated by the multiplexing/demultiplexing unit 130 in the OLT 100, multiplexed into one upstream signal, and transmitted to a relay network 300 side. On the other hand, a downstream signal from the relay network 300 side to each ONU 200 is separated into downstream signals 1 to m for the OSUs 120 of OSU #1 to OSU #m by the multiplexing/demultiplexing unit 130 based on destination ONU information written in the downstream signals and OSU information regarding belonging ONUs 200. The separated downstream signals 1 to m are transmitted to the ONUs 200 at wavelengths of λ1d to λmd of OSUs #1 to #m. Although the downstream signals are broadcast with the wavelengths of the OSUs 120, transmission and reception wavelengths of an ONU 200 are set to transmission and reception wavelengths of the OSU 120 to which the ONU 200 belongs, and thus the ONU 200 selects information destined to the ONU itself from signals of received wavelengths and outputs the selected information to a communication apparatus in a user's home.
The dynamic wavelength bandwidth allocation circuit 110 includes a DWBA calculation unit 112, a switching instruction signal generation unit 111, a control signal transmission unit 113, and a request signal reception unit 114. The request signal reception unit 114 receives a signal including a bandwidth request transmitted from each ONU 200 via each OSU 120, the DWBA calculation unit 112 calculates transmission times and transmission durations of an upstream data signal and a request signal allocated to each ONU 120 based on the request, the switching instruction signal generation unit 111 generates an instruction signal storing information thereof, and the control signal transmission unit 113 transmits the instruction signal to each ONU 200 via each OSU 120. Also, the DWBA calculation unit 112 manages connection information between the OSUs 120 and the ONUs 200 connected to one power splitter or wavelength router 150. When the wavelength is switched, with respect to an ONU 200 that has changed the wavelength, the DWBA calculation unit 112 instructs the multiplexing/demultiplexing unit 130 to change the transfer destination OSU 120 of the downstream signal of which destination is the ONU 200.
FIG. 12 is an example in which OSUs #n+1 to #m are arranged in another OLT #2 and OLT #1 and OLT #2 are arranged at physically different positions, unlike FIG. 1. It is possible to allow a user's communication to continue by wavelength switching if another station building is safe even for a large-scale disaster, such as a case in which the overall station building suffers from a disaster, by arranging the OSUs 120 at physically different positions in this manner. At this time, because it is necessary to transfer ONU control of OLT #1 to OLT #2 or transfer ONU control of OLT #2 to OLT #1, a link for a control signal to be used for transmitting and receiving the control signal is connected between the dynamic wavelength bandwidth allocation circuit 110 of OLT #1 and the dynamic wavelength bandwidth allocation circuit 110 of OLT #2. Although FIG. 12 shows a link for directly connecting OLT #1 and OLT #2, OLT #1 and OLT #2 may be connected via the relay network 300. In the configuration of FIG. 12, it is anticipated that each of fiber lengths between the power splitter or wavelength router 140 provided on the OLT 100 side and OSUs #1 to #n is different from each of fiber lengths between the power splitter or wavelength router 140 and OSUs #n+1 to #m.
FIG. 2 shows the configuration of the ONU. The ONU is configured to include a data reception unit 211, a data transmission unit 223, an upstream buffer memory 212, a downstream buffer memory 222, a destination analysis selection reception unit 221, a frame transmission control unit 213, a frame assembly transmission unit 214, a wavelength-tunable optical transceiver 201, a required bandwidth calculation unit 205, a request signal transmission unit 204, an instruction signal reception unit 202, and a wavelength switching control unit 203.
An upstream signal from a user is received by the data reception unit 211 and temporarily stored in the upstream buffer memory 212. The frame transmission control unit 213 transmits the upstream signal to the frame assembly transmission unit 214 in accordance with a transmission time and transmission duration of the upstream signal designated by an instruction signal. The frame assembly transmission unit 214 configures a frame having a format necessary for transmitting a signal to the OLT in the WDM/TDM-PON configuration and transmits the frame to the wavelength-tunable optical transceiver 201. The wavelength-tunable optical transceiver 201 converts the frame into an optical signal of one of wavelengths λ1u to λmu designated by the wavelength switching control unit 203 and transmits the optical signal to the OLT 100. The wavelength-tunable optical transceiver 201 selects the designated wavelength and receives a downstream signal from the OSU 120 and the destination analysis selection reception unit 221 analyzes the destination of the downstream signal, selects only information destined to the ONU itself, and stores the selected information in the downstream buffer memory 222. The data transmission unit 223 transmits the information stored in the downstream buffer memory 222 as a downstream signal for the user.
The wavelength-tunable optical transceiver 201 receives the instruction signal from the OLT 100, converts the instruction signal into an electrical signal, and transmits the electrical signal to the instruction signal reception unit 202. The instruction signal reception unit 202 analyzes instruction content of the instruction signal, and transmits a switching destination wavelength and a switching execution instruction to the wavelength switching control unit 203 as wavelength switching control at a designated time if a wavelength switching instruction, a wavelength after switching, and a switching start time are included in the instruction signal. The wavelength switching control unit 203 switches the wavelength of the wavelength-tunable optical transceiver 201 in accordance with the wavelength switching control.
Also, the OLT 100 receives information of a bandwidth requested by the ONU 200 from the ONU 200 and uses the received information in allocation of the bandwidth. There are various methods therefor, and, for example, the OLT 100 instructs the ONU 200 to transmit information of the required bandwidth to the OLT 100 using the instruction signal, and the ONU 200 writes the information of the required bandwidth for the OLT 100 in a request signal in accordance with the instruction. In this case, when the instruction signal reception unit 202 receives an instruction for requesting transmission of a request signal, the instruction signal reception unit 202 instructs the request signal transmission unit 204 to generate the request signal. The request signal transmission unit 204 instructs the required bandwidth calculation unit 205 to calculate the required bandwidth. The required bandwidth calculation unit 205 measures an amount of data of an upstream signal stored in the upstream buffer memory 212, determines a required bandwidth amount based on the amount of data, and transmits the required bandwidth amount to the request signal transmission unit 204. The request signal transmission unit 204 generates a request signal in which the required bandwidth amount is written and transmits the request signal to the frame transmission control unit 213.
The instruction signal may include information of a transmission start time and transmission duration of a request signal. In this case, the instruction signal reception unit 202 transmits the information of the transmission start time and the transmission duration of the request signal included in the instruction signal to the frame transmission control unit 213, the frame transmission control unit 213 transmits the request signal to the frame assembly transmission unit 214 at an instructed time, and the frame assembly transmission unit 214 transmits the request signal to the OLT 100 via the wavelength-tunable optical transceiver 201. Also, the instruction signal transmitted from the OLT 100 includes the transmission start time and the transmission duration with respect to transmission of an upstream signal from the user side, which is received by the ONU 200, to the OLT. The request signal transmission unit 204 transmits information of the transmission start time and the transmission duration of the upstream signal included in the instruction signal to the frame transmission control unit 213, the frame transmission control unit 213 extracts a frame of the upstream signal from the upstream buffer memory 212 at the instructed time and transmits the frame to the frame assembly transmission unit 214 for only a period of the transmission duration, and the frame assembly transmission unit 214 transmits the frame to the OLT 100 via the wavelength-tunable optical transceiver 201.